This invention relates to a charged particle accelerating assembly that utilizes a plurality of linearly spaced electrostatic quadrupoles to focus and accelerate charged particles in a beam. More particularly, the invention relates to such an accelerating assembly that has structural and operating parameters which are effective to maintain essentially laminar flow of a beam of particles as it traverses the electrostatic fields of the quadrupoles in the assembly.
The basic operating principles and general structural features of charged particle accelerating assemblies or columns, such as those now commonly used in linear accelerators, or so-called Linacs, are well known to those familiar with high energy physics. Both magnetic and electrostatic quadrupole focusing in such accelerators has been successfully implemented. Electrostatic quadrupoles are particularly advantageous because they consume very little power and are inexpensive to construct, compared with magnetic quadrupoles. Reference may be made to U.S. Pat. Nos. 4,392,080, issued July 5, 1983 and 4,350,927, issued Sept. 21, 1982, for a fairly detailed discussion of electrostatic quadrupole design parameters. Along with a general background description of a method and apparatus for effecting quadrupole focusing, those patents disclose a method and apparatus for accelerating parallel beams of charged particles to produce a beam high intensity.
As is pointed out in those patents, it is usually desirable in the design and construction of a particle accelerating assembly to maximize the attainable beam "brightness", or 6-dimensional phase space density, of the particle beam being focused or accelerated. For the purpose of understanding the invention disclosed herein, the concept of "brightness" can be considered to be a parameter that increases with increasing beam current density and decreases with any tendency of the beam to diverge. One technique for improving beam brightness is to make the beam have laminar flow; thus, the desirability of designing and constructing particle beam acceleration columns to have essentially laminar beam flow is well known. However, a suitable means of attaining the desired objective of an essentially laminar flow beam was not heretofore clearly established.
In the design of early particle beam accelerator columns, the focusing characteristics were typically derived experimentally from columns already in existence, for which acceptable focusing conditions had been determined by trial and error. Because of this emperical approach, the focusing performance of a given accelerator assembly is not generally separable from the particular ion source or other particle injector used with the accelerator. Moreover, so long as the design of future accelerator columns is based primarily on extrapolation from imperically obtained data, a considerable degree of uncertainty will remain in the optimization attainable for the laminar flow of particle beams that are focused and accelerated in such columns.
The traditional way in which particle beams are accelerated in electrostatic devices is based on use of Pierce-type beam accelerating structures. Such assemblies operate well in a space charge limited condition, and will produce particle beams having temperatures that are comparable with those of their originating sources of particles. However, it has been recognized that the current density of a particle beam focused with a Pierce-type accelerator assembly or column is restricted by the Child-Langmuir relation. Thus, a disadvantage of the traditional Pierce-type of acceleration is that if the ion source itself is not the limiting constraint, then the achievable current density is limited by the electric field at which sparking occurs, according to the following equation: ##EQU1## where J is current density in amperes/meter.sup.2, E is the electric field in the accelerating column, V is the terminal voltage and A is the atomic weight of the singly charged ions. It is readily apparent from equation (1) that the achievable current density J decreases as the terminal voltage V is made higher. This limitation can be overcome or avoided, by using electrostatic quadrupole focusing to achieve particle beam acceleration.
In that regard, it can be shown that the space charge limited current density in a constant energy quadrupole transport channel is greater than the density (J) given by equation (1), if it is assumed that the electic fields on the respective quadrupoles is made as high as the electrostatic ion source extraction fields. In practice, that is a conservative assumption. Consequently, it follows that if a particle beam can be transported a large distance at the Child-Langmuir current density limit, the beam can be accelerated as it goes from one quadrupole to another. Hence, the need for having a high gradient acceleration column can be completely avoided.